ZnS:Cu, Co was developed in the 1940s and is one of the earliest commercialized long persistent phosphors to be used in display applications. Similar materials, such as alkali earth sulfides, were also discovered in the 1970s to have long persistence and were termed “Lehmann phosphors”. Defect-induced trapping mechanisms were studied to improve the persistence times using techniques such as co-doping ions to increase trap populations. Examples include modifying CaS:Eu2+ to CaS:Eu2+, Tm3+, or CaS:Bi3+ to CaS:Bi3+, Tm3+. With codopants, persistence times were increased from tens of minutes to over an hour. Also in the 1970s, some alkali earth aluminates, such as SrAl2O4:Eu2+, were found to have a persistent afterglow. See, for example, Abbruscato, V. J. Electrochem. Soc. 1971, 118, 930-933
In 1990s, the discovery of SrAl2O4:Eu2+, Dy3+ and CaAl2O4:Eu2+, Nd3+ was notable since the persistent times were extended an order of magnitude. See, for example, Matsuzawa, T.; Aoki, Y.; Takeuchi, T.; Murayama, Y. J. Electrochem. Soc. 1996, 143, 2670-2673 and Yamamoto, H.; Matsuzawa, T. J. Lumin. 1997, 72-74, 287-289. Soon afterwards, a limited number of materials were found with similar properties. Current research focuses on modifying the color of the phosphor's emission. To date, persistent phosphors excited with solar radiation emit from blue to orange.
The design of a long persistent phosphor typically follows a route that begins with finding a proper host material and activator that emits in the designed wavelength, followed by identifying proper co-dopants to populate traps, and finally to manipulate ground state distance to the host conduction band to achieve solar pumping. In this patent application, long persistent near infrared phosphors have been developed following such a route.
Persistent infrared (IR) phosphors have been studied for only about five years. An initial material was a visible red persistent phosphor CaS:Eu2+, Tm3+, Y3+ that emitted as 650 nm. Since some applications require an infrared persistence, the phosphor Y3Ga5O12:Cr3+ (abbreviated YGG) was developed. This system was improved by incorporating Dy3+ to improve the population traps for the system. Since the YGG system emits at 710 nm and can be seen with the unaided eye, La3Ga5O12:Cr3+ was developed with an emission at 730 nm. Following this discovery, many gallium garnet and gallium germanium/silicate garnet hosts, such as La3Ga5GeO14:Cr3+, Dy3+, were synthesized to have long persistence times with ultraviolet (302/365 nm) and deep ultraviolet excitation (254 nm). See, for example, Jia, D.; Lewis, L. A.; Wang, X. Electrochemical and Solid-State Letters 2010, 13(4), J32-J34. To further shift the emission to 900 nm, La3In2Ga3O12:Cr3+, Dy3+ was developed.
Since the above systems are optimally excited with a 254 nm source, research into designing solar pumped persistent IR phosphors has continued. ZnGa2O4:Cr3+ is the first initial solution that was discovered to satisfy all of these requirements. Following the discovery of persistent ZnGa2O4:Cr3+ emission, Nd3+ is incorporated as an optimal codopant that can extend the persistence of Cr3+ emission. This was further modified to Zn0.96Ga1.8In0.2O4:3% Cr3+,1% Nd3+ to push the emission further into the infrared. Silicon can also be introduced to the system (Zn0.96Ga1.8In0.2SiO6:3% Cr3+,1% Nd3+) to yield a transparent glass.
In order to achieve solar pumping using a persistent Cr3+ emission, the host bandgap is modified to bring the Cr3+ ground state closer to that of the host conduction band. This is accomplished by replacing part of the Ga and Si with Sb and Te into the matrix. An example of this strategy is Zn0.96SbGaTe0.1O4.15N0.05:3% Cr3+,1% Nd3+.